Focus error signal generation using a birefringent plate with confocal detection

ABSTRACT

An improved focus error signal generator device and an optical data delivery and detection system including an optical lens disposed in the path of a return read beam and a birefringent plate disposed in the path of the return beam of light after the optical lens, wherein the birefringent plate provides for a first and second focal plane of corresponding first and second polarization. A pinhole is disposed in the path of the return read beam after the birefringent plate and in close proximity to first and second focal planes. A polarizing beam splitter is positioned after the second focal plane and serves to split the return read beam into two light beams of first and second polarization. First and second detectors are disposed in the path of corresponding first and second polarization light beams. The detectors are connected to the inputs of an electrical differencing circuit that has an output to an optical head servo system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the generation of focus error signals for use in the auto-focussing of optical data storage and retrieval systems. More specifically, the present invention relates to a method and apparatus for generating focus error signals based upon creating two focal planes in a return read beam using a birefringent plate, filtering the read beam using a confocally configured pinhole and deriving a focus error signal related to the difference in distance between the resulting points of focus within the focal planes and the pinhole.

2. Background

As a prerequisite to successful optical data storage, the optical head containing the focussing optics must be positioned properly within the storage layer of the optical storage medium. When recording or retrieving optical data it is essential that the optical head be positioned precisely at the desired storage point Proper positioning of the optical head is typically carried out through auto-focusing techniques implemented by a servo system within the optical head. The signals that drive the auto-focussing process are generated by Focus Error Signal (FES) generator devices that are incorporated into the overall scheme of the optical delivery and detection system.

FES generator devices within optical delivery and detection systems have typically only been required to provide signals in instances where data storage is limited to a single or to a few layers within the optical storage medium. Current technology is generally limited to performing optical data storage on a minimal number of layers within the optical storage medium In most instances, these layers have, generally, about 60 micron separation between adjacent layers. When such a pronounced distance separates the layers, the current FES generator devices are sufficient because layer separation does not present an issue.

However, as technological advances in data storage are made, the capability presents itself to store data on numerous layers within the storage medium. See for example U.S. patent application Ser. No. 09/016,382 filed on Jan. 30, 1998, in the name of inventor Hesselink et al. (assigned to the assignors of the present invention) entitled “Optical Data Storage by Selective Localized Alteration of a Format Hologram” for a detailed discussion of layer definition by format hologram grating structures. That disclosure is hereby expressly incorporated herein by reference as if set forth fully herein.

When data storage is performed on multiple layers the distance separating such layers is minimized. As the separation between the layers shrinks to the about 3 to about 10 micron ranges, the ability to separate out these layers during focus error signal detection becomes more of a concern. The prior art methods are not capable of delineating between layers that are packed so closely together. The present invention serves to provide an FES generator device and a method for FES generation that is capable of differentiating the layers in optical storage medium that have numerous tightly packed layers separated at distances comparable to the Rayleigh range of the illuminating beam. Additionally, the FES generator device and the method of FES generation described herein can be used with a data storage device having multiple storage layers residing at discrete media layers spaced at distances that can be comparable to or substantially longer than the Rayleigh range.

BRIEF DESCRIPTION OF THE INVENTION

Briefly, and in general terms, one embodiment of the present invention comprises an improved focus error signal generator device including an optical lens disposed in the path of a return read beam and a birefringent plate disposed in the path of the return beam of light after the optical lens, wherein the birefringent plate provides for a first and second focal plane of corresponding first and second polarization. A pinhole is disposed in the path of the return read beam after the birefringent plate and in close proximity to first and second focal planes. A polarizing beam splitter is positioned after the second focal plane and serves to split the return read beam into two light beams of first and second polarization. First and second detectors are disposed in the path of corresponding first and second polarization light beams. The detectors are connected to the inputs of an electrical differencing circuit that has an output to an optical head servo system.

Another aspect of the present invention comprises a method for focus error signal generation including the steps of focussing a return read light beam, providing for a birefringent plate disposed in the path of the read light beam that results in a first and second focal plane of corresponding first and second polarizations, providing for a pinhole disposed in the path of the read light beam after the birefringent plate and in close proximity to the first and second focal planes, providing for a polarizing beam splitter disposed in the path of the read light beam after the second focal plane that splits the read light beam into first and second polarization light beams, providing for first and second detectors, respectively, in the paths of the corresponding first and second polarization light beams, and generating a focus error signal related to the difference between the output of the first detector and an output of the second detector.

Additionally, another embodiment of the present invention comprises an optical data delivery and detection system comprising a laser source emitting a light beam, an optical head that receives the light beam, optical lenses within the optical head that focus the light beam on an optical storage media, a data detector that receives the light beam on the beam's return path and provides data signals and a focus error generator device as described herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is an illustration of a simple format hologram grating structure being written by exposing holographic storage medium to two beams of mutually coherent light.

FIG. 1B is an illustration of a complex format hologram grating structure having layer definition generated by superimposing two planar hologram gratings whose vectors are perpendicular to the storage medium surface.

FIGS. 2A and 2B are cross sectional views of format hologram grating structures having two and four constituent holograms, respectively, for track and layer definition.

FIGS. 3A and 3B are cross sectional views of the optical storage devices containing the format hologram grating structures depicted in FIGS. 2A and 2B, respectively.

FIG. 4A is a schematic diagram of an optical head of an optical delivery and detection system in relation to a grating envelope of a depth only format hologram grating structure.

FIG. 4B is a schematic drawing of an optical head of an optical delivery and detection system in relation to a grating envelope of a depth and radial format hologram grating structure.

FIG. 5 is a schematic drawing of an optical delivery and detection system in relation to a grating envelope of a depth only format hologram grating structure.

FIG. 6 is a schematic drawing of a focus error signal generator device, in accordance with a presently preferred embodiment of the present invention.

FIG. 7A is an illustration of an example of ideal positioning of the point of focus in relation to the pinholes, in accordance with a presently preferred embodiment of the present invention.

FIG. 7B is an illustration of an example of point of focus positioning in relation to the pinholes when the light beam is focussed too deeply within the storage medium, in accordance with a presently preferred embodiment of the present invention.

FIG. 7C is an illustration of an example of point of focus positioning in relation to the pinholes when the light beam is focussed too shallowly within the storage medium, in accordance with a presently preferred embodiment of the present invention.

FIG. 8A is a graphical representation of signal intensity versus depth of optical focus for two detectors, in accordance with a presently preferred embodiment of the present invention.

FIG. 8B is a graphical representation of the strength of the focus error signal versus depth of optical focus, in accordance with a presently preferred embodiment of the present invention.

FIG. 9 is a schematic drawing of an optical head showing the general effect on the light beams when the head is not focused properly.

FIGS. 10A, 10B and 10C, respectively, are graphical representations of the strength of the reflected signal as a function of the vertical and transverse dimension within the optical storage medium; the strength of the reflected signal as a function of transverse dimension for two adjacent layers within the optical storage medium; and, the strength of the reflected signal as a function of the vertical dimension for two adjacent track positions within the optical storage medium respectively.

DETAILED DESCRIPTION OF THE INVENTION

Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons from an examination of the within disclosure.

Referring to FIG. 1A, simple format volume hologram 10 is written by exposing a holographic material to two beams of mutually coherent light. In particular, two plane waves incident from opposing sides generate a planar reflection hologram, whose orientation and spatial frequency are governed by the wavelength and angles of incidence of the beams. For example, beam 12 and beam 14 are incident on opposite sides of the material. For optimal reflection, the hologram is Bragg-matched at the readout to a range of angles within the cone of the focused readout beam. This property is the basis of data storage by selective localized alteration of a format hologram grating structure. Confocal detection isolates the light reflected from the focus. The selective localized alteration serves to change the reflectivity at the waist of a focused beam that can be measured using confocal detection; in this manner, data are represented by the localized changes in reflectivity of the format hologram. A variety of complex format holograms can be generated, including layer definition by superimposing two planar hologram gratings whose vectors are perpendicular to the medium surface, as shown schematically in FIG. 1B.

FIGS. 2A and 2B and corresponding FIGS. 3A and 3B are presented here as examples of storage medium that have multiple storage layers, closely spaced, at distances comparable to the Rayleigh range of the illuminating beam. In particular, FIGS. 2A and 2B and corresponding FIGS. 3A and 3B illustrate multiple storage layers within a bulk storage medium. The Rayleigh range storage media that is similar to the one shown here will benefit greatly from the Focus Error Signal (FES) Generator device of the present invention. Further, the present invention will also perform with storage medium having multiple storage layers residing at separate depths in the storage media spaced at distances that can be substantially longer than the Rayleigh range. The Rayleigh range of an illuminating beam is defined as the depth of focus, i.e. the distance over which the point of focus is within twice its minimum diameter. The Rayleigh range is well known by those of ordinary skill in the art.

Referring now to FIG. 2A and FIG. 2B, format hologram grating structures having complex structures for layer and track definition are shown. FIG. 2A shows a cross sectional view of format hologram grating structure 20 having two constituent holograms for track and layer definition. The radius of the format hologram grating structure is represented by horizontal axis 22 and the depth of the format hologram grating structure is represented by vertical axis 24. FIG. 2B shows format hologram grating structure 30 having four constituent hologram gratings for track and layer definition. These constituent holograms exist throughout the entire volume and locally interfere to produce a reflection hologram grating structure with a spatially varying envelope, as shown in FIGS. 2A and 2B. Methods for generating such format hologram grating structures are omitted from this disclosure in order to avoid overcomplicating the disclosure. For a detailed disclosure of generating two and four constituent format holograms and format hologram grating structures see, for example, U.S. Patent application Ser. No. 09/016,382 filed on Jan. 30, 1998, in the name of inventors Hesselink et. al. entitled “Optical Data Storage by Selective Localized Alteration of a Format Hologram” and U.S. patent application Ser. No. 09/229,457 filed on Jan. 12, 1999, in the name of inventors Daiber et. al., entitled “Volumetric Track Definition for Data Storage Media Used to Record Data by Selective Alteration of a Format Hologram”.

FIG. 3A is a cross-sectional view of storage device 40 corresponding to the format hologram grating structure of FIG. 2A. A transparent top cover layer 42 and a transparent bottom cover layer 44, that are typically formed from glass or a polymeric material, enclose holographic storage medium 46 in FIG. 2A. FIG. 3A shows the envelope of the local index perturbation of holographic storage medium 46, for which the carrier frequencies have been removed. Generally a reflected signal from a focused beam will be strongest when it is centered on a peak.

FIG. 3B is a cross-sectional view of storage device 50 corresponding to the format hologram grating structure of FIG. 2B. A transparent top cover layer 52 and a transparent bottom cover layer 54 serve to enclose holographic storage medium 56. Similar to FIG. 3A, FIG. 3B shows the envelope of the local index perturbation of holographic storage medium 56, for which the carrier frequencies have been removed. The magnitude profiles depicted in FIGS. 3A and 3B are a general indication of the expected reflectivity for a high numerical aperture, diffraction-limited beam focused into the holographic storage medium. The features of the format hologram grating structure and the relative thickness of the storage layer and cover layers are not to scale. The number of layers illustrated here is by way of example.

FIGS. 2A and 2B and corresponding FIGS. 3A and 3B illustrate format hologram grating structures that can be utilized for focusing and tracking using a variety of data writing methods, including data writing by selective alteration of the structure itself and data writing by affecting an optical property of the material without substantially affecting the underlying hologram grating structure.

Referring now to FIG. 4A, a schematic of the optical head 62 component of an overall optical delivery and detection system 60 is shown. The optical head 62 can be moved radially, as shown by arrow 64, and in depth, as shown by arrow 66, to access different portions of a holographic storage medium 68. The illustrated holographic storage medium 68 has a format hologram grating structure stored therein. Other details of the structure of optical head 62, used for measuring intensity at a particular depth, are not necessary for an understanding of the present invention and are omitted to avoid overcomplicating this disclosure. The construction of an optical head for measuring the intensity at a particular depth of the material can be found in U.S. Patent Application Ser. No. 09/016,382 filed on Jan. 30, 1998, in the name of inventor Hesselink et. al., entitled “Optical Data Storage by Selective Localized Alteration of a Format Hologram”, previously expressly incorporated herein by reference.

In the case of a disk, rotation brings different angular portions of holographic storage medium 68 into optical communication with optical head 62. In the case of a medium formed in card or tape (not shown in FIG. 4A), linear motion brings different lateral portions of the holographic storage medium to the optical head. FIG. 4A illustrates the position of optical head 62 in relation to a grating envelope of a depth only format hologram grating structure. The top layer 70 and the bottom layer 72 of holographic storage device 74 can comprise glass or a polymeric material. As a function of the vertical position of the focus, the reflected intensity is greatest when the focus is positioned at the center of a layer, and least when positioned between the layers. By way of example, focus servo can be achieved by wobbling the head vertically, measuring the change in intensity, and directing the head to be positioned where the reflection is highest.

FIG. 4B illustrates an optical head 62 in relation to a grating envelope of a depth and radial format hologram grating structure. Layer selection may be achieved using the same simple focus servo; a tracking servo can be similarly achieved by wobbling optical head 62 transversely, measuring the change in intensity, and directing the head to be positioned where the reflection is highest.

FIG. 5 illustrates an optical delivery and detection system 80. Laser 82 illuminates beam 84 that is expanded by beam expansion optics 86 and directed towards holographic storage medium 88. Once the beam is expanded it passes through polarizing beam splitter 90 where it is directed towards first corner turning mirror 92 located within optical head 94 and then directed towards objective lens 96. Variable spherical aberration correction (SAC) optics 98 can be used in conjunction with objective lens 96 to focus on different layers within holographic storage medium 88. The use of SAC optics 98 is particularly important when trying to reach depths within storage medium 88 in excess of approximately 100 microns. On the beam's forward and return paths, light passes through quarter wave plate 100 that serves to change the polarization of the beam The use of polarizing beam splitter 90 and quarter wave plate 100 serves to increase the efficiency of the overall system 80. Alternatively, a standard 50/50 beam splitter can be used in place of polarizing beam splitter 90 and quarter wave plate 100. Once the polarization has been changed, the beam passes through polarizing beam splitter 90. The polarizing beam splitter 90 recognizes the change in polarization and directs the beam towards several operational paths. A fraction of the light is split in the direction of each path. A standard 50/50 -beam splitter 102 directs a portion of the light in data path 104 towards data detector 106 and additional optional detectors (not shown in FIG. 4). The portion of light not directed toward data path 104 is directed towards second 50/50 beam splitter 108 that serves to direct a portion of the light in tracking path 110 toward tracking error detector 112. The portion of light not directed toward tracking path 110 is directed towards optional second corner turning mirror 114. Finally, the light is reflected off corner turning mirror 114 in auto-focus path 116 toward a focus error detector 118.

A presently preferred embodiment of the present invention is illustrated in FIG. 6. Shown in FIG. 6 is an improved focus error signal (FES) generator device 120 comprising lens 122, birefringent plate 124 and pinhole 126. An optical head (not shown in FIG. 6) focuses light onto a data layer in a data storage medium. The data storage medium, by way of non-limiting example, may be comprised of either multiple layers of data storage or a bulk, monolithic holographic storage medium that has a format hologram grating stored therein to define layers. Read beam 128 enters FES generator device 120 on a return path from the data storage medium. Read beam 128 passes through first optical lens 122 that serves to focus read beam 128. First optical lens 122 is typically formed from glass or a polymeric material. First optical lens 122 should have a working distance that slightly exceeds the distance between first optical lens 122 and birefringent plate 124.

Once read beam 128 passes through first optical lens 122 it is focused towards birefringent plate 124. Birefringent plate 124 characteristically has two indices of refraction that result in two separate focal planes (not shown in FIG. 6) associated with two corresponding polarizations 140 and 142. Generally first polarization 140 and second polarization 142 will be separated by 90° degrees. Each focal plane has a corresponding point of focus, shown in FIG. 6 as first point of focus 130 and second point of focus 132. By example, birefringent plate 124 may be formed from crystal calcite or any other birefringent material may be used to form birefringent optical plate 124. The amount of shift in the indices of refraction may be an appropriate consideration when choosing the birefringent material that will form birefringent optical plate 124.

FIG. 6 shows, by way of example, birefringent plate 124 adjacent to plate 134 with pinhole 126 formed therein. FES generator device 120 is not limited to a configuration having birefringent plate 124 adjacent to plate 134. FES generator device 120 may be configured so that birefringent plate 124 is located at any distance between first optical lens 122 and plate 134. In the illustrated embodiment, plate 134 may be physically attached to birefringent plate 124 using, for example, an adhesive compound or any other fastening mechanism. Additionally, a mounting mechanism may be implemented when birefringent plate 124 and plate 134 are located in close proximity to one another.

Pinhole 126 serves to confocally filter read beam 128 as it departs birefringent plate 124. Pinhole 126 is, typically, formed in a glass plate that is coated with a metal foil. The size of pinhole 126 is dependent upon the working distance of first optical lens 122, the diameter of read beam 128 and the wavelength of read beam 128. By way of example, pinhole 126 may be sized so that 90% of intensity of the read beam passes through the pinhole when the pinhole is positioned at a point of focus. The pinhole may typically range in size from about 1 micron in diameter to about 50 microns in diameter, and is typically about 8 microns in diameter. Those skilled in the art will realize that the pinhole can be formed in a variety of manners. The pinhole configuration that is shown here is by way of example only and is not intended to be in any way limiting.

After read beam 128 has been filtered through pinhole 126 and resulting first point of focus 130 and second point of focus 132 have formed, read beam 128 is transmitted through second optical lens 136. Second optical lens 136 is, typically, a standard glass or polymeric lens. Second optical lens 136 serves maximize the performance of polarizing beam splitter 138 by directing read beam 128 towards polarizing beam splitter 138. Second optical lens 136 is an optional element of FES generator device 120. It is possible and within the inventive concept herein disclosed to configure FES generator device 120 without second optical lens 136 in instances where the focal length and beam diameter of read beam 128 are such that the use of second optical lens 136 would not be required.

Polarizing beam splitter 138 separates the light of first polarization 140 from the light of second polarization 142. The use of a polarizing beam splitter is well known by those of ordinary skill in the art. Light of first polarization 140 is directed towards detector 144 and light of second polarization 142 is directed towards detector 146.

Detector 144 and detector 146 may be standard photodiodes that serve to convert light beam photons to electrical signals in the form of electrons (an electrical current). The electrical signal is then provided to electronic differencing circuit 148 that generates a focus error signal. A servo system (not shown in FIG. 6) uses the focus error signal to direct the position of the objective lens in the optical head, thus completing the auto-focus procedure. Servo systems are well known in the art.

In contrast, in a prior-art data detection arm (shown in FIG. 5 at 106), a pinhole is generally used for confocal depth selection, and is generally situated so that light reflected from the desired depth —the focus of the read beam —comes to a focus in the plane of the pinhole, passing efficiently through it For the FES generator device 120 shown in FIG. 6, when the optical head is focused on a layer within the storage medium first polarization 140 will form first point of focus 130 before pinhole 126 and second polarization 142 will form second point of focus 132 after pinhole 126.

The birefringent plate 124 and plate 134, having pinhole 126 formed therein, are characteristically mounted within a FES generator device housing (not shown in FIG. 6) with the aid of a precision alignment fixture. Use of such alignment fixtures to position elements, such as pinhole plates or birefringent plates, are well known by those of ordinary skill in the art.

The positioning of the points of focus in relation to the pinhole is instrumental in determining proper auto-focus. The ideal positioning of the points of focus are shown in FIG. 7A. This ideal positioning is achieved when the optical head is properly focused on the desired layer within the storage medium with respect to the depth of a data layer or layer center. First polarization 140 has first point of focus 130 slightly before pinhole 126 and second polarization 142 has second point of focus 132 slightly after pinhole 126. For this configuration, the intensities detected for first polarization 140 and second polarization 142 are equivalent

FIGS. 7B and 7C illustrate light beams that are focused either too deeply or too shallowly within the storage medium with respect of the depth of a layer or layer center. FIG. 7B gives an example of focusing too deeply within the storage medium. If the optical head is focused too deeply into the medium, first point of focus 130 and second point of focus 132 in the FES generator device are shifted towards the detectors (not shown in FIG. 7B). For moderate perturbations, at pinhole 126, the read beam is more tightly focused for first polarization 140 and less tightly focused for second polarization 142. Therefore, more light is transmitted via first polarization 140 than second polarization 142 and, thus, more light is detected by the detector associated with first polarization 140 than the detector associated with second polarization 142.

FIG. 7C provides an example of the optical head being focused too shallowly into the medium, resulting in first point of focus 130 and second point of focus 132 being shifted away from the detectors (not shown in FIG. 7C). For moderate perturbations, at pinhole 126, the read beam is more tightly focused for second polarization 142 and less tightly focused for first polarization 140. Therefore, more light is transmitted via second polarization 142 than first polarization 140 and, thus, more light is detected by the detector associated with second polarization 142 than the detector associated with first polarization 140. Thus, a simple difference in the intensities of the light indicates in which direction the optical head is out of focus.

A simple differencing circuit can be employed to generate a focus error signal in the present invention. For example, a first detector A outputs a current which is converted to a voltage A, and a second detector B outputs a current that is converted to a voltage B. Current-to-voltage techniques are well known in the art. The focus error signal (A−B) is positive when the focus is too deep, and negative when the focus is too shallow. Furthermore, the signal (A−B) is stronger as the beam is further out of focus. The focus error signal can be further normalized to the total strength (A+B) in order to compensate e.g. for fluctuations in laser strength. The focus error signal can be used to position the optical head so that it focuses on a layer e.g. by defining the minimum and maximun tolerable value limits D1<(A−B)÷(A+B)<D2; where D1 is the minimum limit and D2 is the maximum limit. When the normalized focus error signal (A−B)÷(A+B) falls outside this range, the head is moved in the appropriate direction to bring it within this range, thus focusing on a particular layer.

Referring now to FIG. 8A, signal intensities for first detector A and second detector B are shown as a function of depth of the material. The vertical line 220 indicates a particular depth of a formatted layer. At this depth, first detector A and second detector B provide the same output voltage that corresponds to the same signal strength. Referring now to FIG. 8B, the value of the focus error signal (A−B) is shown as a function of depth of the material. If the beam from the optical head is focused too deeply with respect to the nearest layer, then the focus error signal (A−B) is positive, and therefore the normalized focus error signal (A−B)÷(A+B) is positive. If the beam from the optical head is focused too shallowly with respect to the nearest layer, then the focus error signal (A−B) is negative, and therefore the normalized focus error signal (A−B) ÷(A+B) is negative. Both the value of the focus error signal (A−B) and its slope can be used together to determine the direction and distance the optical head must be moved to restore focus to the nearest layer. Alternatively, the value of the normalized focus error signal and its slope can be used together to determine the direction and distance the optical head must be moved to restore focus to the nearest layer. Furthermore, the value of the focus error signal (A−B) and its slope can be used to supplement the evaluations based on a normalized focus error signal. Layer selection is accomplished by keeping track of the number of layers crossed as the optical head passes from the first layer it encounters to the others.

The strength of the focus error signal defined by (A−B) in FIG. 8B is proportional to the phase shift of the two signals A and B, illustrated in FIG. 8A. This relationship can be understood by referring to FIG. 9, which shows the general effect on the light beams when the head is not focused on a layer. FIG. 9 does not include all critical parts of the optical head, such as a spherical aberration corrector, but illustrates the effects of out-of-focus head position. For clarity, the figure does not illustrate effects of the index of refraction of the material; these effects would be apparent to those skilled in the art of optical sciences. In FIG. 9, the layer is indicated by a single reflecting surface for illustration purposes only.

Referring to FIG. 9, a light beam 230 passes through the objective lens 232, is reflected by the layer 234, turns through the objective lens 232, passes through the beam splitter 236, and is then focused by the detection lens 238. The focal position of the light returning to the objective lens 232 is shifted by twice the depth positioning error. As would be apparent to those skilled in the optical sciences, the focus position in the detection optics therefore shifts by δz₂=(2δz₁/n)(f₂/f₁)². In this equation n is the index of refraction of the material, f1 is the focal length of objective lens 232, f2 is the focal length of the detection lens 238,δz₁ is the depth positioning error, and δz2 is the focal shift in the FES generator. An axial pinhole displacement δz2 therefore results in a detected phase shift of δz₁=(n/2)δz₂(f₁/f₂)² as illustrated in FIG. 8A.

Equivalently, the condition for strongest focus error signal can be cast in terms of the pinhole placement with respect to the layer spacing imaged by the detection lens. The strongest focus error signal (A−B) is achieved when the axial pinhole displacement is set such that the displacement of the pinhole from the point of focus for polarization A is adjusted to optionally detect a depth positioning error of δz_(1A)=(¼)Δz, and for polarization B is adjusted to optionally detect a depth positioning error of δz_(1B)=−(¼)Δz, where Δz is the spacing of the layers as measured by the optical focus displacement (FIG. 8B). For data detection, however, it is generally preferred that the pinhole reside equal distance from the points of focus of the beam, for which the displacement would be less than the Rayleigh range of the optical focus in the material (e.g. δz_(1A)=−δz_(1B=(){fraction (1/2 )})z_(R), where Z_(R) is the Rayleigh range of the beam focused in the material). Thus, it is generally preferred that the data detection and focus error generation optics described herein are in separate arms/paths (as shown in FIG. 5).

Equivalently, the condition for strongest focus error signal can be cast in terms of the pinhole placement with respect to the layer spacing imaged by the detection lens 238. In these terms, the strongest focus error signal (A−B) is achieved when the axial pinhole displacement from the point of focus for polarization A is δz_(2A)=(¼)Δz′, and for polarization B is δz_(2B)=−(¼)Δz′, where Δz′=(2Δz/n)(f₂/f₁)² which is the spacing of the layers as measured by the optical focus displacement (FIG. 9B). If it is required that the two signals A, B be in focus (e.g. to detect data), then the phase shift should be reduced to less than the Rayleigh range of the optical focus of the detection lens (e.g. δz_(2A)=−δz_(2B)=(½)z_(R)′, where z_(R)′ is the Rayleigh range of the beam focused by the detection lens).

In an alternative preferred embodiment of the present invention, a focus error signal (B−A) can be generated to keep the beam focused on the nulls between data layers. This may be advantageous for data recording that does not utilize selective localized alteration of a format hologram.

Due to the properties of confocal detection, the pinhole diameter should be less than or equal to the diameter of the optical beam reflected from the material for optimal rejection of out-of-focus signals from other layers. In contrast, for maximum signal strength and positioning tolerance, the pinhole diameter should be greater than or equal to the optical beam diameter. A particular choice that balances these constraints is to set the pinhole size to just pass nearly all of the optical intensity of the non-aberrated beam. By way of example, the pinhole size can be selected so that 90% of intensity of the light beam passes through the pinhole when the pinhole is positioned at the beam focus.

The present invention can also be practiced to keep an optical head focused on tubular format regions (tubular format cross-sectional regions are illustrated in the storage medium of FIGS. 2B, 3B and 4B). The round cross sections sweep out tubular format regions throughout the radius of the material that result from complex format hologram grating structures. In this case, the reflected light detected by the system is relatively small between the tubes. Thus, the beam must be kept radially positioned on the data tube in order to remain focused on a particular data layer. Some tracking methods used for single layer and multiple media layer storage devices can be adapted for use in this invention as disclosed. By way of example, one such method of tracking comprises wobbling the focused spot radially, measuring the change in intensity, and directing the head to be positioned where the reflection is highest which correspond to the track centers. The operation of the focus error generator device of the present invention is otherwise the same.

Referring now to FIGS. 10A, 10B and 10C, data are shown for the strength of light reflected off a format hologram grating structures that defines layers and tracks which together define tubular format regions. FIG. 1OA shows the strength of the reflected signal as a function of the depth (vertical dimension) and the radius (transverse dimension) within the storage medium at a fixed angular position. FIG. 1OB shows the strength of the reflect signal as a function of radius (transverse dimension) for two adjacent layers within the storage medium. Note that adjacent tracks are shifted in phase by half a period. FIG. 10C shows the strength of the reflected signal as a function of depth (vertical dimension) for two adjacent track positions. Note that these layers may be placed as closed as only a few times the depth of field of the readout optical beam.

Although illustrative presently preferred embodiments and applications of this invention are shown and described herein, many variations and modifications are possible which remain within the concept, scope and spirit of the invention, and these variations would become clear to those skilled in the art after review of this disclosure. The invention, therefore is not limited except in spirit of the appended claims. 

What is claimed is:
 1. An apparatus for generating a focus error signal from a read light beam retuning from an optical storage media comprising: a first optical lens disposed in the path of said read light beam; a birefringent plate disposed in the path of said read light beam after said first optical lens, said birefringent plate creating a first focal plane of a first polarization and a second focal plane of a second polarization for said read light beam; a pinhole disposed in the path of said read light beam after said birefringent plate and in close proximity to said first and second focal planes; a polarizing beam splitter disposed in the path of said read light beam after said second focal plane, said polarizing beam splitter creating a first polarization beam and a second polarization beam from said read light beam; a first optical detector disposed in the path of said first polarization beam; a second optical detector disposed in the path of said second polarization beam; and an electrical differencing circuit having a first input coupled to said first optical detector, a second input coupled to said second optical detector and an output.
 2. An apparatus in accordance with claim 1, wherein said electrical differencing circuit generates a focus error signal related to the difference between the output of said first optical detector and the output of said second optical detector.
 3. An apparatus in accordance with claim 1, wherein said electrical differencing circuit generates a focus error signal related to the difference between the distance from said first focal plane to said pinhole and the distance from said second focal plane to said pinhole.
 4. An apparatus in accordance with claim 1, further comprising: a second optical lens disposed in the path of said read light beam after said second focal plane and before said polarizing beam splitter.
 5. An apparatus in accordance with claim 1, wherein said pinholes has a diameter that allows for the passage of about 90% of the intensity of said read light beam when said pinhole is positioned at a point of focus.
 6. A method for generating a focus error signal from a read light beam returning from an optical storage media comprising the steps of: focussing said read light beam; providing a birefringent plate through which said read light beam passes to create a first focal plane of a first polarization and a second focal of a second polarization; providing a pinhole in the path of said read light beam after said birefringent plate and in close proximity to said first and second focal planes; providing a polarizing beam splitter disposed in said path of said read light beam after said second focal plane, said polarizing beam splitter creating a first polarization beam and a second polarization beam from said read beam; providing a first optical detector in the path of said first polarization beam; providing a second optical detector in the path of said second polarization beam; and generating a focus error signal related to the difference between an output from said first optical detector and an output from said second optical detector.
 7. A method in accordance with claim 6, wherein the step of generating a focus error signal further comprises: generating a focus error signal related to the difference between the distance from said first focal plane to said pinhole and the distance from said second focal plane to said pinhole.
 8. A method in accordance with claim 6, further comprising the steps of: providing an optical lens through which said read light beam passes after said second focal plane and before said polarization beam splitter.
 9. A method in accordance with claim 6, further comprising the steps of: positioning said pinhole after said first focal plane; and positioning said pinhole before said second focal plane.
 10. A method in accordance with claim 6, further comprising the steps of: providing said pinhole with a diameter that allows for passage of about 90% of the intensity of said read light beam when said pinhole is positioned at a first point of focus within said first focal plane.
 11. A method in accordance with claim 6, further comprising the steps of: providing said pinhole with a diameter that allows for passage of about 90% of the intensity of said read light beam when said pinhole is positioned at a second point of focus within said second focal plane.
 12. A system for optical delivery and detection comprising: a laser source that emits a first light beam; an optical head that receives said first light beam; an optical lens within said optical head that focuses said first light beam on a location within a storage media; a data detector that receives said first light beam returning from said storage media and provides data signals; a tracking error detector that receives said first light beam returning from said storage media and provides a tracking error signal to said optical head; and a focus error signal generator that receives said first light beam returning from said optical storage media and provides a focus error signal to said optical head, said focus error signal generator including: a first optical lens disposed in the path of said first light beam; a birefringent plate disposed in the path of said first light beam after said first optical lens, said birefringent plate creating a first focal plane of a first polarization and a second focal plane of a second polarization from said first light beam; a pinhole disposed in the path of said first light beam after said birefringent plate and in close proximity to said first and second focal planes; a polarizing beam splitter disposed in the path of said first light beam after said second focal plane, said polarizing beam splitter creating a second light beam of first polarization and a third light beam of second polarization from said first light beam; a first optical detector disposed in the path of said second light beam; a second optical detector disposed in the path of said third light beam; and an electrical differencing circuit having a first input coupled to said first optical detector, a second input coupled to said second optical detector and an output.
 13. A system in accordance with claim 12, wherein said storage media has layer separation (Δz), said layer separation having a corresponding spacing (Δz′) as measured by an optical focus displacement and wherein said pinhole is placed (¼)Δz′ from said first focal plane and said pinhole is placed −(¼)Δz′ from said second focal plane. 